1. Field of the Invention
The present invention pertains to interval timers and, more particularly, to two-wire electronic devices designed to be retrofitted, that is, to be substituted in place of two-wire or two-terminal conventional wall switches.
2. Background Information
In electronic designs that have been proposed heretofore for interval timers adapted for the aforenoted retrofitted application, it is generally desirable to have the timer operable in a simple two-wire configuration. However, this presents a serious problem because the voltage supply for the various components involved in the control circuit must be available while the main switch is conducting.
To obtain the required control circuit voltage for operation of the aforenoted control devices or components, the main, electronic switch is usually turned off for a fraction of a cycle. This allows sufficient voltage to bias the control circuit components.
Referring for the moment to FIG. 1, a conventional interval timer is therein shown. This timer includes a pilot or control thyristor designated TRIAC 2, to drive the main, thyristor switch, TRIAC 1, which directly controls the load.
Although simple in design, the conventional timer of FIG. 1 produces a DC voltage offset at the switch terminals A and B. This can be appreciated by reference to FIG. 2 in which typical wave forms are depicted. The problem that is presented is that if the load is in the form of magnetic devices such as transformers, ballasts, and motors, such devices will be extremely sensitive to such DC voltage offsets. As a consequence, magnetic core saturation and high excitation currents will be produced. Therefore, the method embodied in the conventional timer of FIG. 1 is applicable only to resistive type loads.